1. Field of the Invention
The present invention relates to a flash photography system and an electronic flash device for use in the system which transmits a flash readiness signal and a flash light control termination signal from the electronic flash device to a camera and switches the camera photography mode and/or its indication in accordance with the signals.
2. Description of the Prior Art
A Japanese Laid-Open Patent Application No. Sho 54-24625 discloses a flash photography system wherein an electronic flash device generates a flash readiness signal upon detecting that its main capacitor is fully charged, i.e., that it is ready to fire and when the flash readiness signal is transmitted to a camera, the camera is automatically switched to a flash photography mode which controls the shutter at a shutter speed tuned to the flash of the electronic flash device and, at the same time, an indication of the camera being switched to the flash photography mode is given in the camera viewfinder. That flash photography system also transmits a flash light control termination signal from the electronic flash device, upon termination of automatic flash light control, to the camera and indicates, in response to the signal, the flash light control termination in the camera viewfinder. In the system, both the flash readiness signal and the flash light control termination signal are transmitted from the electronic flash device to the camera through the separate terminals. However, providing the separate terminals between the electronic flash device and the camera for transmission of the two signals is not desirable in that it increases not only cost but also the frequency of terminal contact failure. Therefore, a desirable system is that it usually transmits a flash readiness signal from an electronic flash device to a camera through one terminal and transmits a flash light control termination signal through the same terminal for a predetermined period of time only upon termination of automatic flash light control so that re-transmission of the flash readiness signal is possible after the lapse of the predetermined period. Thus it reduces the number of terminals between the electronic flash device and the camera. However, such a system poses a new problem. That is, only the flash light control termination signal is transmitted during the predetermined period of time upon termination of automatic flash light control by the electronic flash device, making it impossible for the camera to judge whether the main capacitor of the electronic flash device is fully charged, i.e., whether the electronic flash device is ready to fire. In other words, since the same terminal is used for transmission of the two signals, the camera cannot identify a signal appearing at the terminal even if it is a flash light control termination signal and judges instead that the electronic flash device is ready to fire. Therefore, in case another shooting is performed for repeated photography in short cycles during the predetermined period of time when a flash light control termination signal is transmitted, the camera misjudges that the electronic flash device is ready to fire before it is so and actuates switching to the flash photography mode. The result is an underexposed picture.
U.S. Pat. No. 4,349,260 discloses a flash photography system which eliminates the drawbacks of the above prior art system by use of a common terminal for the transmission of the flash readiness signal and the flash light control termination signal and a circuit for interrupting the generation of the flash light control termination signal in response to a camera release operation. This proposed system, however, employs a dedicated electronic flash device having no light receiving element for the flash light control, thereby necessitating a stop signal transmission from a camera to the flash device. In other words, the flash device is useful only for a camera capable of light measurement during an exposure operation and having an additional terminal for transmission of the stop signal for automatic flash light control.